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I read that genetic drift is random variation in the relative allele frequencies of a population.

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This question however seems to pin genetic drift down to increasing deleterious allele frequencies. Is the answer to this question correct? I don't think that increase of a deleterious allele necessarily indicates that genetic drift is operating; it suggests genetic drift, because one would think that such alleles should decrease in frequency rather than increase, but random chance in a large population can also increase the frequency of deleterious alleles.

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Drift is a random element of evolution from one generation to the next, it is equally likely to spread a deleterious mutation as a beneficial one.

The reason the answer is D is because selection should remove a deleterious allele, while drift makes no distinction, and is therefore the only scenario where drift is expected to be a reasonable answer. However, it is possible for drift to contribute to all of those scenarios.

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  • $\begingroup$ Personally I would prefer that the question you show there said probably rather than definitely - if a deleterious allele is linked to a beneficial one it can be spread by selection despite itself being deleterious $\endgroup$
    – rg255
    Nov 12, 2014 at 18:30

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